It is well known that phyllosilicates, such as smectite clays, e.g., sodium montmorillonite and calcium montmorillonite, can be treated with organic molecules, such as organic ammonium ions, to intercalate the organic molecules between adjacent, planar silicate layers, thereby substantially increasing the interlayer (interlaminar) spacing between the adjacent silicate layers. The thus-treated, intercalated phyllosilicates, having interlayer spacing of at least about 10-20 Angstroms and up to about 100 Angstroms, then can be exfoliated, e.g., the silicate layers are separated, e.g., mechanically, by high shear mixing. The individual silicate layers, when admixed with a matrix polymer, before, after or during the polymerization of the matrix polymer, e.g., a polyamide--see U.S. Pat. Nos. 4,739,007; 4,810,734; and 5,385,776--have been found to substantially improve one or more properties of the polymer, such as mechanical strength and/or high temperature characteristics.
Exemplary of such prior art composites, also called "nanocomposites", are disclosed in published PCT disclosure of Allied Signal, Inc. WO 93/04118 and U.S. Pat. No. 5,385,776, disclosing the admixture of individual platelet particles derived from intercalated layered silicate materials, with a polymer to form a polymer matrix having one or more properties of the matrix polymer improved by the addition of the exfoliated intercalate. As disclosed in WO93/04118, the intercalate is formed (the interlayer spacing between adjacent silicate platelets is increased) by adsorption of a silane coupling agent or an onium cation, such as a quaternary ammonium compound, having a reactive group which is compatible with the matrix polymer. Such quaternary ammonium cations are well known to convert a highly hydrophilic clay, such as sodium or calcium montmorillonite, into an organophilic clay capable of sorbing organic molecules. A publication that discloses direct intercalation (without solvent) of polystyrene and poly(ethylene oxide) in organically modified silicates is Synthesis and Properties of Two-Dimensional Nanostructures by Direct Intercalation of Polymer Melts in Layered Silicates, Richard A. Vaia, et al., Chem. Mater., 5:1694-1696(1993). Also as disclosed in Adv. Materials, 7, No. 2: (1985), pp, 154-156, New Polymer Electrolyte Nanocomposites: Melt Intercalation of Poly(Ethylene Oxide) in Mica-Type Silicates, Richard A. Vaia, et al., poly(ethylene oxide) can be intercalated directly into Na-montmorillonite and Li-montmorillonite by heating to 80.degree. C. for 2-6 hours to achieve a d-spacing of 17.7 .ANG.. The intercalation is accompanied by displacing water molecules, disposed between the clay platelets with polymer molecules. Apparently, however, the intercalated material could not be exfoliated and was tested in pellet form. It was quite surprising to one of the authors of these articles that exfoliated material could be manufactured in accordance with the present invention.
Previous attempts have been made to intercalate polyvinylpyrrolidone (PVP), polyvinyl alcohol (PVA) and poly (ethylene oxide) (PEO) between montmorillonite clay platelets with little success. As described in Levy, et al., Interlayer Adsorption of Polyvinylpyrrolidone on Montmorillonite, Journal of Colloid and Interface Science, Vol. 50, No. 3, March 1975, pages 442-450, attempts were made to sorb PVP (40,000 average M.W.) between monoionic montmorillonite clay platelets (Na, K, Ca and Mg) by successive washes with absolute ethanol, and then attempting to sorb the PVP by contact with 1% PVP/ethanol/water solutions, with varying amounts of water, via replacing the ethanol solvent molecules that were sorbed in washing (to expand the platelets to about 17.7 .ANG.). Only the sodium montmorillonite had expanded beyond a 20 .ANG. basal spacing (e.g., 26 .ANG. and 32 .ANG.), at 5.sup.+ 30% H.sub.2 O, after contact with the PVP/ethanol/H.sub.2 O solution. It was concluded that the ethanol was needed to initially increase the basal spacing for later sorption of PVP, and that water did not directly affect the sorption of PVP between the clay platelets (Table II, page 445), except for sodium montmorillonite. The sorption was time consuming and difficult and met with little success.
Further, as described in Greenland, Adsorption of Polyvinyl Alcohols by Montmorillonite, Journal of Colloid Sciences, Vol. 18, pages 647-664 (1963), polyvinyl alcohols containing 12% residual acetyl groups could increase the basal spacing by only about 10 .ANG. due to the sorbed polyvinyl alcohol (PVOH). As the concentration of polymer in the intercalant polymer-containing solution was increased from 0.25% to 4%, the amount of polymer sorbed was substantially reduced, indicating that sorption might only be effective at polymer concentrations in the intercalant polymer-containing composition on the order of 1% by weight polymer, or less. Such a dilute process for intercalation of polymer into layered materials would be exceptionally costly in drying the intercalated layered materials for separation of intercalate from the polymer carrier, e.g., water, and, therefore, apparently no further work was accomplished toward commercialization.
In accordance with one embodiment of the present invention, intercalates are prepared by contacting a phyllosilicate with a PVP polymer, preferably essentially alcohol-free, or a PVA intercalant polymer composition, wherein the PVA preferably contains 5% or less residual acetyl groups, more preferably fully hydrolyzed, containing 1% or less acetyl groups.
In accordance with an important feature of the present invention, best results are achieved using an oligomer (herein defined as a pre-polymer having 2 to about 15 recurring monomeric units, which can be the same or different) or polymer (herein defined as having more than about 15 recurring monomeric units, which can be the same or different) composition for intercalation having at least about 2%, preferably at least about 5% by weight intercalant oligomer or intercalant polymer concentration, more preferably about 50% to about 80% by weight oligomer and/or polymer, based on the weight of oligomer and/or polymer and carrier (e.g., water and/or other solvent for the intercalant oligomer or intercalant polymer) to achieve better sorption of the intercalant polymers between phyllosilicate platelets and so that less drying is required after intercalation. The oligomer or polymer sorbed between silicate platelets that causes separation or added spacing between adjacent silicate platelets and, for simplicity of description, both the oligomers and polymers are hereinafter called the "intercalant" or "intercalant polymer" or "polymer intercalant". In this manner, the water-soluble oligomers or polymers will be sorbed sufficiently to increase the interlayer spacing of the phyllosilicate in the range of about 10 .ANG. to about 100 .ANG., for easier and more complete exfoliation, in a commercially viable process, regardless of the particular phyllosilicate or intercalant polymer.
In accordance with the present invention, it has been found that a phyllosilicate, such as a smectite clay, can be intercalated sufficiently for subsequent exfoliation by sorption of polymers or oligomers that have carbonyl, hydroxyl, carboxyl, amine, amide, and/or ether functionalities, or aromatic rings to provide metal cation chelate-type bonding between two functional groups of one or two intercalant polymer molecules and the metal cations bonded to the inner surfaces of the phyllosilicate platelets. Sorption and metal cation electrostatic attraction or bonding of a platelet metal cation between two oxygen or nitrogen atoms of the molecules; or the electrostatic bonding between the interlayer cations in hexagonal or pseudohexagonal rings of the smectite layers and an intercalant polymer aromatic ring structure increases the interlayer spacing between adjacent silicate platelets or other layered material to at least about 10 .ANG., and preferably at least about 20 .ANG., and more preferably in the range of about 30 .ANG. to about 45 .ANG.. Such intercalated phyllosilicates easily can be exfoliated into individual phyllosilicate platelets before or during admixture with a thermoplastic or thermosetting matrix polymer to form a matrix polymer/platelet composite material, or nanocomposite, having one or more properties substantially improved in comparison with the matrix polymer alone.